added new pow_mod method and mult_pow!

Improves the old version since multiplication was able to overflow more easily!

1 files changed, 17 insertions, 7 deletions

diff --git a/math/modExp.cpp b/math/modExp.cpp
index f738268..863ff4e 100644
--- a/math/modExp.cpp
+++ b/math/modExp.cpp
@@ -1,7 +1,17 @@
-ll modPow(ll b, ll e, ll p) {
-	if (e == 0) return 1;
-	if (e == 1) return b;
-	ll half = modPow(b, e / 2, p), res = (half * half) % p;
-	if (e & 1) res *= b; res %= p;
-	return res;
-}
\ No newline at end of file
+//0<=a,b <=n and n <= MAX(ll)/2
+ll mult_mod(ll a, ll b, ll n) {
+	if(a == 0 || b == 0) return 0;
+	if(b == 1) return a % n;
+	
+	if(b % 2 == 1) return (a + mult_mod(a, b-1, n)) % n;
+	else return mult_mod((a + a) % n, b / 2, n);
+}
+
+//0<=a,b<=n and n <= MAX(ll)/2
+ll pow_mod(ll a, ll b, ll n) {
+	if(b == 0) return 1;
+	if(b == 1) return a % n;
+	
+	if(b % 2 == 1) return mult_mod(pow_mod(a, b-1, n), a, n);
+	else return pow_mod(mult_mod(a, a, n), b / 2, n);
+}